The invention relates to the construction and operation of superconducting rotating machines, including superconducting electric motors.
Superconducting air core, synchronous electric machines have been under development since the early 1960""s. The use of superconducting windings in these machines has resulted in a significant increase in the field magnetomotive forces generated by the windings and increased flux and power densities of the machines. These early superconducting machines included field windings wound with low temperature superconductor (LTS), originally with NbZr or NbTi and later with Nb3Sn. The field windings were cooled with liquid helium from a stationary liquifier. The liquid helium was transferred into the rotor of the machine and then vaporized to use both the latent and sensable heat of the fluid to cool the windings. This approach proved to be viable for only very large synchronous motors and generators. With the advent of high temperature superconductor (HTS) in the 1980""s, investigations ensued to determine the feasibility of HTS windings in superconducting synchronous machines.
The invention features a superconducting rotating machine having a relatively compact construction while still providing a relatively high output power. In effect, the construction provides a superconducting rotating machine having an increased power density characteristic. The superconducting rotating machine is of the type having a stator assembly and a rotor assembly which rotates within the stator assembly and is spaced from the stator assembly by a gap.
In one aspect of the invention, the rotor assembly of the superconducting rotating machine includes at least one HTS superconducting winding assembly which, in operation, generates a magnetic flux linking the stator assembly, a refrigeration system for cooling the at least one superconducting winding of the rotor assembly and the superconducting rotating machine has a gap shear stress characteristic in a range between 30 lbs/in2 (psi) and 100 lbs/in2. For example, in one embodiment, the superconducting rotating machine (e.g., motors or generators) has a gap shear stress characteristic of about 45 psi.
Gap shear stress is an effective measure of the torque density of a machine. It relates machine performance to the surface area in the gap between the rotor assembly and stator assembly. In particular, gap shear stress is numerically equivalent to the machine torque divided by the area and radius of the gap. If the rotor experiences a surface shear stress equal to the gap shear stress, a torque equal to the design torque would be transmitted to the shaft of the machine.
Embodiments of this aspect of the invention may include one or more of the following features.
The superconducting rotating machine has a specific power in a range between 1.5 kilowatts/kilogram (kw/Kg) and 4.5 KW/Kg, for example, 2.0 KW/Kg. The superconducting rotating machine has a power density in a range between 1.2 Mwatts/m3 and 4 Mwatts/m3 for an 1800 rpm rotating machine. It is appreciated that as the speed of the rotating machine becomes larger or smaller, the specific power and power density will become proportionally larger or smaller as well.
The superconducting winding assembly includes a superconducting coil having a superconductor tape wound about and disposed along an axis of the winding assembly to provide a plurality of concentric turns defining an opening. The opening has a dimension which gradually decreases, in the direction along the axis, and from a first end to a second end of the winding assembly. Each turn of the superconductor tape has a broad surface maintained substantially parallel to the axis of the winding assembly.
The decreasing dimension opening defined by the winding configuration of the coil provides a coil having a tapered profile. The advantages of a tapered superconducting coil having this arrangement are numerous. For example, the tapered superconducting coil is well-suited for use in applications where the coil is to be positioned in annularly-shaped volumes, such as those commonly found in rotating electric machines. In general, the tapered arrangement eliminates stepped profiles, common with other stacked arrangements. In particular, the tapered superconducting coil requires relatively fewer stacked individual coils to fill annularly-shaped volumes. This is in contrast to other superconducting coil assemblies, which require stacking of many more thin, individual coils to fill an annularly-shaped volume. Moreover, reducing the number of individual coils, in turn, reduces the number of electrical connections between the individual coils, thereby increasing the overall performance and reliability of a coil assembly using tapered coils. In addition, the superconductor tape of the present invention is wound with its broad surface maintained substantially parallel to the axis of the coil (as well as to adjacent turns.) This feature is particularly advantageous when the tape is formed of less flexible, brittle materials, such as ceramic-based high temperature superconducting materials. Furthermore, the tapered configuration provides better critical current (Ic) retention characteristics and allows for better coil grading.
In certain embodiments, the superconductor tape is wound in a racetrack shape defining a pair of opposing arcuate end sections and a pair of opposing substantially straight side sections. The superconductor tape includes a multi-filament composite superconductor having individual superconducting filaments which extend the length of the multi-filament composite conductor and are surrounded by a matrix-forming material.
The superconductor tape includes an anisotropic high temperature superconductor, for example, Bi2Sr3Ca2Cu3O. Alternatively, the anisotropic high temperature superconductor is a member of the rare-earth-copper-oxide family.
The rotor assembly includes a cylindrical support member for supporting the superconducting winding assembly.
The cylindrical support member includes end extension members defining warm/cold transition regions. The cylindrical support member is formed of a high strength, low thermal conductivity composite material, for example, a G-10 phenolic or woven-glass epoxy. Thus, the low thermal conductivity composite material thermally isolates the cryogenically-cooled superconducting coils from the outside ambient temperature world. The rotating machine further includes an axially compliant member for radially supporting an end of the cylindrical support member.
The refrigeration system includes a cryocooler located in a stationary reference frame, and a closed circulation system external to the cryocooler interfacing the stationary reference frame with a rotating reference frame in which the superconductor winding assembly is located. Among other advantages, the refrigeration system of the invention permits the cryocooler to remain stationary while eliminating the need for an extensive sealing system needed to flow coolant through an open circulation system.
The closed circulation system includes a heat transfer assembly located in the rotating reference frame and a heat transfer gap defined between the cryocooler and the heat transfer assembly. The heat transfer assembly transfers heat from the superconducting winding assembly to the heat transfer gap. Thus, the heat transfer gap provides an efficient structure for transferring heat from the superconductor winding to the cryocooler. A coolant (e.g., helium, neon, nitrogen, or oxygen) is located in the heat transfer gap.
In one embodiment, the rotating heat transfer assembly includes a heat pipe having a first fluid path for directing a flow of liquid coolant from a cold end to a warm end of the heat transfer assembly, and a second fluid path for directing a flow of gas coolant from the warm end to the cold end of the heat transfer assembly.
The superconducting rotating machine further includes a warm end conduction block and a cold end conduction block, which define the warm end and cold end of the heat transfer assembly, respectively. The warm end conduction block and cold end conduction block are both mounted to the heat pipe. The warm end conduction block is further mounted to the superconducting winding assembly. The cold end conduction block includes first fins and the cryocooler includes second fins rotatable with respect to the second fins and intermeshed with the first fins. The space between the intermeshed fins define the heat transfer gap.
The rotor assembly includes induction structure for carrying current at levels sufficient to allow a transient induction mode of operation. Because induced currents are generated in the rotor assembly in the induction mode, a structure for supporting these currents is necessary. The induction structure is configured to allow the superconducting motor to generate a starting torque which is at least 50% of the rated torque in the induction mode of operation. Further, the induction structure is configured to allow the superconducting motor to generate a peak torque (breakdown torque) which is at least twice the rated torque in the induction mode of operation.
In one embodiment, at least a portion of the induction structure is spaced from the at least one superconducting winding by a thermal isolation vacuum region. That is, a portion of the induction structure is in the warm region of the rotor assembly, such as an electromagnetic shield member. The electromagnetic shield member includes a conductive, non-magnetic material (e.g., copper, aluminum).
The induction structure can also include a cryostat positioned between the thermal isolation vacuum region and the electromagnetic shield member. Thus, the cryostat not only serves to cool the superconducting windings of the rotor assembly, but also serves to support induced currents when the motor operates in the induction mode.
The cold cylindrical support member which supports the at least one superconducting winding can also serve as part of the induction structure.
In certain embodiments, the superconducting electric motor also includes an adjustable speed drive for providing an adjustable frequency electrical signal to the stator assembly.
The superconducting rotating machine also includes an exciter, having a radially laminated rotatable disk including AC windings, and a stationary disk also including AC windings. The stationary disk is axially spaced from the radially laminated, rotating disk to form a gap therebetween. In essence, the rotating disk and stationary disks and coils together provide a transformer to induce AC voltage and current in the rotating coil.
The exciter further includes a rectifier coupled to the AC windings in the rotor and having an output coupled to the DC windings. The superconducting rotating machine further includes a frame for supporting the stationary disk, rectifier and current regulator.
The stator assembly includes a cylindrical support tube having a bore extending along a longitudinal axis of the support tube and a single-layer winding wound along the axis of the support tube. The cylindrical support tube is formed of an electrically resistive composite material including, for example, glass and epoxy. The stator assembly further includes a cooling member (e.g., at least one helically wound tube), thermally coupled to an external surface of the winding, and having at least one passage extending therethrough for receiving a coolant from an external source. The cooling member includes helically wound tubes, a first one of the helically wound tubes disposed between the outer surface of the support tube and an inner surface of single layer winding. A second one of the helically wound tubes is thermally coupled to an outer surface of one of the single layer windings. The windings of the stator are radially spaced from a longitudinal axis of the stator and are circumferentially spaced from each other, with alternate ones of the windings having end regions which extend radially away from the axis. The cooling member further includes an end region helically wound tube that is thermally coupled to the radially-extending end regions. The helically-wound tubes are formed of a non-magnetic material.
The stator assembly includes an outer banded member disposed around the superconducting winding and formed of a high permeability material. In one embodiment, the outer banded material is a steel wire wound around the at least one superconducting winding. This banded member is wound under tension to load the stator assembly against the stator bore tube.
The stator assembly includes an outer housing for enclosing the cylindrical support tube, the single-layer winding, and the outer banded material. The stator assembly also includes an encapsulating material (e.g., adhesive epoxy) surrounding the cylindrical support tube, the single-layer winding, and the outer banded material adhesive within the outer housing.
Other advantages and features of the invention will become apparent from the following description and the claims.